This invention relates to novel copolyesters and hot melt adhesives formed therefrom.
Various problems have been associated with the use of adhesives for plasticized vinyl substrates such as vinyl wall coverings, vinyl tile, edge banding and other vinyl laminations. For instance, over a period of time the plasticizers from the highly plasticized vinyl substrate tend to migrate to the adhesive/substrate interface, and even into the adhesive itself. Attack of the adhesive interface by the plasticizer results in a rapid loss in adhesion, and attack of the adhesive itself by the plasticizer results in a loss of cohesion or internal strength of the adhesive, accompanied by splitting and transfer.
Conventional hot-melt pressure sensitive adhesives such as ethylene/vinyl acetate copolymers and rubber based adhesives are rapidly attacked by the migrating plasticizers from the vinyl substrate resulting in a decrease in the tensile peel values of the adhesive bond to very low levels and a loss of cohesive strength of the adhesive. Accompanying the migration of plasticizer from the vinyl substrate is a rapid shrinkage of the vinyl substrate by as much as 5 to 10%. While the acceptable degree of vinyl shrinkage or decrease in peel strength will, of course, vary with the application, typical requirements for the pressure sensitive adhesives utilized in connection with vinyl substrates are a 40-150 ozs./linear inch peel strength (or bond value) with less than a 30% decrease after aging for one week at 70.degree.C, and less than 0.5% vinyl shrinkage, with no adhesive splitting or transfer, after similar aging. Additional requirements for a hot melt pressure sensitive adhesive in general are good thermal stability (i.e., the ability to resist degradation or gelation at high temperature), good wetting characteristics, low melt viscosity, and compatibility with a large variety of tackifying resins, modifiers, pigments and the like; in some instances, it is furthermore desirable that these adhesives be applicable to a substrate from a solution. Additional requirements for a hot melt non-pressure sensitive adhesive in general are high heat resistance (i.e., the ability to form an adhesive bond which withstands high temperatures) and cold resistance (i.e., the ability to form an adhesive bond which withstands low temperatures).
Copolyesters of ethylene glycol, terephthalic acid and dimer acid and similar copolyesters have been described in various patents such as U.S. Pat. Nos. 3,383,343; 3,390,108; 3,329,740 and French Pat. No. 1,398,551. Such copolyesters have not proven to be especially useful as adhesives for vinyl substrates despite their low shrinkage levels, because they are not highly resistant to plasticizer migrating from the vinyl substrate. Typically the peel strength decreases by at least 50% on aging, with accompanying adhesive transfer and splitting. Attempts to improve the adhesive properties of such copolyesters by chain extending the copolyesters with various non-polymeric polyols such as trimethylol propane and various polyether polyols such as trimethylol propane/ethylene oxide adducts have not proven successful. Another feature inhibiting the use of such polyesters has been the cost and difficulty in manufacturing. In conventional polyester manufacture, high temperatures (up to 280.degree.C), high vacuum (1.0-0.1 mm Hg) and long polycondensation times are required to obtain products exhibiting useful adhesive characteristics. Such extreme conditions do not always result in the best obtainable products due to degradation reactions occurring during polycondensation, and of course increase the cost of manufacture.
Accordingly it is an object of the present invention to provide a novel copolyester which is a high molecular weight chain-extended aromatic dicarboxylic acid/dimer acid/glycol copolyester useful as a hot melt adhesive.
Another object is to provide such an adhesive which inhibits shrinking of a vinyl substrate and is resistant to plasticizer migrating from the vinyl substrate.
A further object is to provide such an adhesive exhibiting a 40-150 ounces/linear inch peel strength with a maximum 30% decrease in strength after aging and a maximum 0.5% shrinkage of the vinyl substrate with no adhesive splitting or transfer after aging.
It is also an object to provide for pressure sensitive applications such an adhesive exhibiting good thermal stability, good wetting characteristics, low melt viscosity, and a high compatibility with a large variety of tackifying resins, modifiers, pigments, etc.
It is another object to provide for non-pressure sensitive applications such an adhesive exhibiting high heat and cold resistance.
It is a further object to provide a method of manufacturing such polyesters under very mild conditions involving temperatures no higher than 270.degree.C, pressures of 1-30 mm Hg and relatively short polycondensation times of 2-6 hours.